The present invention relates to an input, or as the case may be input-output management circuit for a process control system in an industrial or tertiary sphere such as a programmable automaton.
As is known, programmable automata include input and output circuits managed by a central processing unit including a processor and associated memories, particularly an input-output image memory as well as a data bus providing a connection between the input and output circuits and the central unit; in particular, the input circuits include a plurality of parallel input channels able to be connected to respective sensors, the state of the input channels being cyclically scrutinzed by the central unit.
The input circuit include components interconnected together and fixed to a card whose surface must be as small as possible; in addition, the input circuits are directly subjected to parasites and noise affecting the terminals and conductors connecting with the sensors.
Generally in the input circuit optocouplers are used associated with input channels so as to provide isolated transmission of the signals coming from the sensors. There exists one optocoupler per input channel and the optocouplers therefore occupy considerable space on the input card. Furthermore, different drawbacks result from the use of optocouplers: their phototransistor in fact risks being saturated should a violent disturbance occur due to a common mode parasite; furthermore, the dispersion and lack of fidelity in the characteristics of optocouplers mean that the disabled time thereof is badly known and furthermore varies in time.
The purpose of the invention is to overcome these drawbacks due to the use of optocouplers for each of the input channels without adversely affecting the behavior of the process control system to disturbances, and this by means of a compact input architecture with good performances as far as the safety of acquisition of the input signals is concerned.
The invention also makes it possible to check the voltage of the sensors in a simple way.